1. Field of the Invention
The present invention relates to a drive controller which utilizes an electromechanical transducer.
2. Description of Related Art
In the related art, various actuators which utilize a piezoelectric element have been proposed. For example, an actuator which utilizes a piezoelectric element is proposed for driving a lens of a camera for focusing (refer to Japanese Patent Publication No. 2633066). In the actuator, it is general that the supply of driving pulses to the piezoelectric element is controlled in such a way as to cause an extension speed and a contraction speed along a direction of the piezoelectric element to differ, so as to cause the piezoelectric element to extend and contract, whereby a lens driving shaft member is caused to extend and contract in the direction, so that a lens which is in frictional engagement with the shaft member is caused to move minutely along the direction.
In the actuator, however, since the shaft member is caused to extend and contract along the direction by causing the piezoelectric element to extend and contract in such a way that the extension speed and the contraction speed along the direction of the piezoelectric element are caused to differ, so as to move minutely the lens which is in frictional engagement with the shaft member, it has been difficult to move the lens by a distance proportional to the number of driving pulses.
A technique is disclosed in JP-A-11-356070 in which in order to increase the accuracy with which a lens is driven for focusing, a moving amount of the lens is detected to obtain a moving speed of the lens, and a time to stop the supply of driving pluses for driving the lens is controlled so that the moving speed of the lens becomes a predetermined target speed.
Incidentally, there is known a technique in which while moving a lens in a direction by a number of driving pulses for each movement, an AF evaluation value (for example, a contrast value) is measured which is obtained from an image photographed via the lens to obtain a position where the AF evaluation value becomes maximum, so that the lens is moved to the position to be in focus. In this technique, since measuring points of AF evaluation values are spaced apart from each other at intervals, in order to implement more accurate focusing, an operation is carried out in which the lens is returned in the reverse direction to a peak position of the AF evaluation value after the measurement of AF values (AF search).
However, even in JP-A-11-356070, nothing is mentioned as to a control for the return operation. In addition, the nature of the piezoelectric element changes while the operation is repeated of returning the lens in the reverse direction to the peak position of the AF evaluation value, whereby the number of pulses which is required to actually move the lens by a specified distance is changed, this making difficult for the AF search to be carried out with higher accuracy.